1. Field of the Invention
This invention relates to a signal demodulator and detector, and more particularly, to an improved FM demodulator and detector.
2. Description of the Prior Art
In the prior art, frequency selective circuits including phase locked loops have been utilized as signal demodulators for recovering the modulating signal of an FM signal. Phase locked loops per se are known in the art as, for example, basically described in the book entitled "Phase Locked Techniques" by Floyd M. Gardner, published by John Wiley & Sons, 1966. Such a phase locked loop has been utilized in discrete systems applications such as aerospace telemetry, and, more recently, has been incorporated into integrated circuits for use in tone detectors, as shown in U.S. Pat. No. 3,659,224, entitled "Temperature Stable Integrated Oscillator," by J. V. Ball and in FM demodulators, as shown in U.S. Pat. No. 3,564,434, entitled "Integrated Frequency Selective Circuit and Demodulator Including Phase Locked Loop," by H. R. Camenzind et al. However, for FM radio systems it is necessary to have both an FM signal demodulator and a reliable FM signal detector which squelches the recovered modulating signal when the FM signal is not present. Such as FM signal demodulator and detector that is provided on an integrated circuit and that operates at relatively low DC supply voltages with relatively low current drain is particularly well adapted for portable FM radio applications where discrete systems would be impractical and prohibitively expensive.
For the foregoing and other shortcomings and problems, there has been a long felt need for an improved FM signal demodulator and detector.